I have 18hp craftsman tractor, single cylinder, 31H777, 0202E1 03. The mower runs for 20-30 mins, sputters and eventually stops. If I wait for it to cool down it will run until engine becomes hot. It starts easy when cold and not so easy when hot, often backfiring. The mower was in storage from 2004-2008. It ran well last summer after I had the carb flushed and new gaskets by local lawnmower mechanic. This year I've changed the fuel filter, changed the air filter, checked the gas cap for clogging and replace for of the gas lines. After reading thru the forum I'm starting to believe the valves maybe out of adjustment. Help is appreciated. scvol